[flang] Handle SAVE attribute and statement

As with COMMON blocks, we can't completely check SAVE statements and
attributes until the end of the specification part when we have seen
full declarations of entities. So when SAVE is specified, add it to one
of the two sets in `saveInfo_`. At the end of the specification part,
check that those entities can have SAVE applied and set it if it is
not already implicitly set (e.g. due to being in a module). Also apply
the "global" SAVE if present (i.e. setting it on every applicable
entity).

Add `IsDummy()` and `IsFuncResult()` to `Symbol` to simplify some of
the checks. Also detect attempts to put a function result in a common
block.

Original-commit: flang-compiler/f18@af19c02baca371ffff13722fa17792a1f26e7bcd
Reviewed-on: https://github.com/flang-compiler/f18/pull/298